1. Field of the Invention
The disclosed invention relates generally to an optical inspection system used for inspecting substrates after photolithography processing, and more specifically to facilitating preventive maintenance of such an optical inspection system.
2. Discussion of the Background
In a manufacturing process of a semiconductor device, a prescribed circuit pattern is formed on the surface of a semiconductor wafer by photolithography. In the photolithography step, a series of treatments are carried out such that a resist film is formed on a cleaned semiconductor wafer by supplying a photoresist solution onto the semiconductor wafer, exposing the resist film to light in a prescribed pattern and subsequently developing the pattern.
Although the chance of misprocessing at any single lithography step is small, a typical wafer goes through 20 to 25 lithography steps. Excursions due to process equipment problems, mishandling, and contamination can occur at each of these steps, so the cumulative probability of a wafer experiencing a yield-limiting defect becomes significant. While some defects impact only a small area of the wafer and may not require rework, other defects impact 30% or more of the wafer. These are defined as global defects. After develop inspection (ADI) procedures detect, classify, and disposition wafers with lithography defects for rework. Typical defects include problems with photo resist or ARC/BARC coating, edge bead processing, exposure, alignment, and development, as well as defects caused by contamination or handling, such as particles or scratches. Each recovered wafer can result in savings of thousands, or even tens of thousands of dollars, in revenue.
Many of economically re-workable defects are macro-scale, and thus manually detectable through visual inspection by trained operators at microscope stations. Since manual inspection is a relatively slow process compared to the photolithography process throughput, automatic inspection has recently been performed by an after develop inspection (ADI) system to improve throughput.—Further, ADI systems can inspect 100% of wafers processed and can detect defects down to 50 μm.
In typical photolithography systems, after various coating, expose, and develop steps, the wafers are delivered to an ADI inspection station that captures a series of whole wafer images using simultaneous dark and bright field illumination. A full-color image of 100% of the wafer is captured and is characterized by its RGB signature. Such a signature has three elements: a red value, a green value, and a blue value that vary within a predetermined range such as 0-255. The resulting image or signature is compared to that of a “golden” wafer with no defect and a confidence score is assigned indicating how similar the signatures are. When a significant difference is detected, further analysis is performed, to classify the defect so that appropriate remedial action can be performed. The RGB signature itself is useful for flagging global defects, but the ADI also can detect “local” defects such as particles, scratches, comets, etc.
The ADI optical hardware primarily consists of a camera and a light source, such as a strobing Xenon or similar type lamp, for example. To ensure accurate inspection results, periodic maintenance of the ADI system must be performed. For example, as the light source degrades over time, the light source requires periodic calibration as well as replacement at the end of the light source's finite life. Such ADI preventive maintenance typically requires that the photolithography system be taken off line. For example, in order to calibrate the light source or determine whether the light source requires replacement, a technician must empty the photolithography system of production wafers and manually run calibration wafers on the system. This results in costly equipment down time while the preventive maintenance is completed.
Conventionally, preventive maintenance of the ADI system has been performed at arbitrary time intervals, such as four week intervals. However, such arbitrary intervals may not coincide with other preventive maintenance actions that require equipment down time, resulting in greater overall down time. Further, the light source may not require calibration at the scheduled interval, such as when the system has been used infrequently during the four week period, which also results in unnecessary tool down time. Still further, the light source may require calibration or replacement prior to the end of the four week period, resulting in inaccurate inspection results for production wafers inspected after the maintenance was actually required.
Finally, even where preventive maintenance of the ADI system is necessary, manual techniques for completing such maintenance are inherently time consuming and, as noted above, require a technician to take the photolithography system off line resulting in costly equipment down time.